A New Approach to Estimate Monthly Groundwater Nitrate Loading from an Individual Agricultural Field

The magnitude and timing of nitrate export from a single agricultural field was investigated using an approach that combined detailed vertical groundwater nitrate concentration profiles with Darcy's flux estimation at the down-gradient edge of the field. Groundwater was sampled approximately monthly using passive diffusion samplers. Darcy groundwater fluxes were estimated using field-measured hydraulic gradients and lab-estimated hydraulic conductivity. Nitrate exported from the agricultural field over the year-long monitoring period was estimated to be 71 kg N ha-1, while the chloride mass flux estimate was 49 kg Cl ha-1. The similarity in field applied chloride (51 kg Cl ha-1) and estimated export suggests the mass flux estimation approach is robust. The highest monthly mass flux was observed in February and March (approximately 9 kg N ha-1) and was associated with leaching from the soil zone with the onset fall precipitation after the growing season.